SECTION IV. 

 C.IROMOGEN1C, PHOTOGENIC, AND THERMOGENIC BACTERIA. 



CHAPTER XII. 



CHROMOPAROUS BACTERIA, PRODUCING RED AND YELLOW 

 COLOURING MATTERS. 



86. Coloured and Colouring- Bacteria. 



IN classifying the chromogenic (colour-producing) bacteria, the situation, as well 

 as the nature, of the colour has an importance that cannot be disregarded. An 

 examination for this first-named characteristic in individual species quickly leads 

 to the differentiation of the chromogenic bacteria into coloured bacteria on the 

 one hand and colouring bacteria on the other the cells in the latter being 

 themselves quite "colourless, but excreting a coloured transformation product; 

 these species have been designated chromoparous by Beyerinck. 



In the coloured bacteria, on the contrary, the colouring matter remains 

 within the cells. This group may be divided into two sub-groups, the one 

 comprising those coloured bacteria the colouring matter of which performs an 

 important physiological function, as in the case of the purple bacteria treated 

 of in the following chapter ; such bacteria are termed chromophorous. On the 

 other hand, the second sub-group includes those coloured bacteria in which the 

 colouring matter has no such function, and must be regarded as a purely passive 

 metabolic product, which is, nevertheless, not excreted (as in the chromoparous 

 species), but remains within the cell without manifesting any apparent activity. 

 These bacteria are termed parachromophorous. 



The chemical properties of the bacterial colouring matters and their 

 importance for distinguishing one species from another were discussed by PAUL 

 SCHNEIDER (I.) after exhaustive experiments with thirty different species. His 

 results in this connection may be thus summarised : (i) The bacterial colouring 

 matters can to some extent be differentiated by their behaviour towards solvents. 

 (2) A given species, grown under identical conditions, always produces the 

 same colouring matter. (3) Two species, differing as regards form and con- 

 ditions of growth, may in certain cases produce the same colouring matter. 

 (4) Most of the species apparently producing the same colouring matter, and 

 also analogous in other respects, can be differentiated by the reactions of their 

 colouring matter. 



87. Micrococcus Prodiglosus. 



This being the oldest known chromogenic bacterium, will be dealt with first. 



Many a victim of the proceedings taken against witchcraft during bygone 

 centuries must have been consigned to the stake on the charge of having fabri- 

 cated the blood-red spots that were occasionally found developed on the Host, 



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